Sides of defile appear steep (4)
I believe the answer is:
dear
'steep' is the definition.
(thesaurus)
'sides of defile appear' is the wordplay.
'sides of' means to remove the middle letters (only the letters on the sides remain).
'appear' becomes 'ar' (I am not sure about this - if you are sure you should believe this answer much more).
'defile' with its middle taken out is 'de'.
'de'+'ar'='DEAR'
(Other definitions for dear that I've seen before include "letter opener" , "Overpriced" , "Treasured" , "polite form of address" , "Beloved - expensive" .)